The present invention relates to power amplifiers for base stations in wireless communication systems and, in particular, to power amplifier control for Multiple Carrier Power Amplifiers (MCPA).
MCPAs presently account for more than half the equipment cost of a base station in a wireless communication system with the cost of a typical MCPA being directly proportional to maximum peak power provided thereby due to the corresponding increase in cost for higher capacity active components, cooling hardware, and the like. Base station costs thus can be correspondingly controlled by reducing or controlling the maximum peak power provided by an MCPA. However, degradation of performance beyond tolerable limits may occur when demands for peak power on traffic channels are at their greatest and exceed the maximum rating of the MCPA. Moreover specific functions which require nominal maximum power to be applied may be affected when MCPAs operate at reduced power levels.
When reducing peak power associated with MCPAs, often times, depending on algorithms used and peak traffic demand patterns, degradation may occur periodically and with differing levels of severity. According to a typical power dimensioning scheme, more carriers may be allocated to a base station than can typically be supported within peak power dimensions, for example, traffic maximums are achieved simultaneously on most or many channels. Presently, MCPA's may be dimensioned for “worst-case”, i.e. full RF power on all carriers. It is thus desirable to dimension the MCPA closer to the average power. Several MCPA control schemes attempt to provided power levels closer to average power. In particular, U.S. Pat. No. 5,384,547 to C. N. Lynk, et al, describes a linear device for attenuating a signal if a power threshold is exceeded. Japanese Patent abstract JP 9139679 A describes a peak power reduction circuit which detects envelope power level for a multicarrier signal and attenuates a predetermined amount for a predetermined time if a threshold is exceeded. Such solutions, however, in themselves, pose additional problems in that if an MCPA is dimensioned close to average needed power, a large negative impact on service quality may be felt, since all channels are affected at overload. Present systems may further experience relatively moderate overall changes in output power, since the power is related only to the number of active carriers. It would be desirable for systems to adapt to more dynamic changes and a lower average/peak ratio due to, for example, dynamic BS power control per carrier or time slot, downlink Discontinuous transmission (DTX), or the like.
Some MCPAs may include autonomous power reduction devices. Overdrive of MCPA's with autonomous devices power reduction normally results in MCPA switch-off and activation of an alarm output. The alarm output may be fed to an alarm printer and, in the case of autonomous MCPA control, service quality may deteriorate to a large degree. Lower carrier power implies more unreliable coverage, slower access times, unreliable handoff.
It may further be desirable for large providers of wireless communications equipment to have independent suppliers of different components associated with such equipment. It is thus desired that MCPA's be capable of reductions in power independently of whether or not, for example, a particular base station is configured to handle such reductions. Independent power reduction capabilities may allow “optimal” power reduction from, for example, an operating cost standpoint, and “optimal” communication system behavior is achieved. Such performance may be achieved, for example, when operating parameters such as power level, are adapted to match all components within particular equipment such that for example the impact of a network service in overload situations is reduced or eliminated.
To effect power reduction, an MCPA and an associated base station may reduce power on carriers and channels associated with the base station. In a typical wireless system such as, for example, a system in accordance with the Global System for Mobile communications (GSM) system governed by the specification contained, for example, in “Digital cellular telecommunications system (Phase 2+); Radio transmission and reception (GSM 05.05 version 6.3.0 Release 1997). In a typical wireless communication system in accordance with, for example, the GSM specification, there are typically two kinds of channels: control channels and traffic channels. Control channels normally transmit at nominal, or maximum, power so that control signals responsible for new call setup can reach all the way to the cell border and within the cell. Accordingly, the power with which the control channel transmits establishes the cell radius. It is further important for the control channel to transmit with maximum power so that extended services such as, for example, SMS service, broadcast/paging services, and the like may be offered. It should be noted that because control channels are assumed to transmit at full power, functions such as, for example, “consistency check” and Locating are made possible. Should transmit power levels associated with the control channel vary, some degradation can be expected.
For traffic channels, transmit power may be set by a base station typically to a level inverse to the expected attenuation. For example, power may be set low when a mobile station (MS) is close to the BS and may be set at a level closer to maximum power when the MS approaches the cell border. Depending on the system different permutations of power management may occur based on location, as described, and based on timeslot. For example, TDMA Cellular PCS standard, ANSI/TIA/EIA-136, published Mar. 22, 1999, by TR-45.3 Committee, does not exclude per-timeslot downlink power regulation although power levels associated with different timeslots on a carrier may not be allowed to differ much. However, it should be noted that in accordance with ANSI/TIA/EIA-136, power may be temporarily set to a maximum level just before to just after a handoff.
In other systems, power level control may be implemented in TDMA based telecommunications systems in a more straightforward manner. In such systems transmit power associated with certain downlink bursts, particularly those which are being transmitted with more power than the corresponding mobile station requires, is adjusted during a given timeslot, and in such a way, that the transmit power adjustment resembles that of a typical fading event, in terms of time of occurrence and rate of occurrence, e.g., dB per msec. In so doing, other mobile stations, using the same frequency carrier or an adjacent frequency carrier, receiving a downlink burst during that timeslot at a power level that is marginally adequate, such as mobile stations operating at or near the border of nearby cells, are better able to cope with the effects of fading, since they are subjected to less interference. For further details, see for example, U.S. patent application Ser. No. 09/475,640 entitled “METHOD AND SYSTEM FOR MEASURING AND REPORTING RECEIVED SIGNAL STRENGTH” filed Dec. 30, 1999, and U.S. patent application Ser. No. 09/399,764 entitled “DOWNLINK TIMESLOT POWER CONTROL IN A TDMA SYSTEM” filed Sep. 21, 1999.
It would therefore be desirable to provide power control in MCPA equipped base stations which would reduce average power to the greatest extent possible while maintaining nominal power for critical functions and acceptable power levels for traffic channels.